1. Field of the Invention
The present invention relates to an image forming apparatus, an image formation system, a density-unevenness correction method and a recording medium.
2. Description of Related Art
In general, an electrophotographic image forming apparatus (such as a printer, a copy machine, and a fax machine) is configured to irradiate (expose) a charged photoconductor with (to) laser light based on image data to form an electrostatic latent image on the surface of the photoconductor. The electrostatic latent image is then visualized by supplying toner from a developing device to a photoconductor drum (image carrier) on which the electrostatic latent image is formed, whereby a toner image is formed. Further, the toner image is directly or indirectly transferred to a sheet, and then heat and pressure are applied to the sheet at a fixing nip to form a toner image on the sheet.
Incidentally, it is known that, in the image forming apparatus, cyclic density unevenness is caused in the sub scanning direction of the image due to the rotation runout of the photoconductor drum and the developing roller (developer bearing member). FIG. 1 illustrates sheet S in which density unevenness of output image S1 is caused.
When such density unevenness is caused, first portion S11 having a high color density and second portion S12 having a low color density are alternately formed in output image S1 when an image composed of one color is output to sheet S as illustrated in FIG. 1, for example. In particular, in the case of density unevenness due to the rotation runout of developing roller, first portion S11 and second portion S12 are formed at short intervals since the diameter of the developing roller is smaller than the diameter of the photoconductor drum. Therefore, when density unevenness due to the rotation runout of the developing roller is caused, the density unevenness tends to be reflected in the image output on the sheet, and consequently precise correction of the density unevenness becomes necessary.
For example, Japanese Patent Application Laid-Open No. 2012-88522 discloses a technique of correcting density unevenness in the sub scanning direction and the main scanning direction based on the rotation cycle of the image bearing member and the detection signal of the density detection section. In this technique, density unevenness is corrected by detecting the density unevenness in the sub scanning direction, determining the correction pattern, and applying the correction pattern at all positions in the main scanning direction.
However, when the positions of the holding parts of the both end portions of the developing roller in the direction of the rotation axis in the developing device are shifted, the developing roller is tilted to the photoconductor drum. FIG. 2 illustrates sheet S in which density unevenness tilted to the main scanning direction is caused. As illustrated in FIG. 2, in this state, when density unevenness is caused due to rotation runout of the developing roller, first portion S11 and second portion S12 in output image S1 are tilted to the main scanning direction (hereinafter referred to as “tilted density unevenness”).
In the configuration disclosed in Japanese Patent Application Laid-Open No. 2012-88522, the same correction pattern is applied at all positions in the main scanning direction. Consequently, elimination of the density unevenness at all of position A, position B, position C and so forth in the main scanning direction (see FIG. 2) cannot be achieved, and density-unevenness correction for the tilted density unevenness cannot be precisely performed.